At present, display devices employing liquid crystals are widely utilized in watches, electronic calculators, etc. For driving such liquid crystal display devices, the optical anisotropy and the dielectric anisotropy of liquid crystal substances are utilized in the devices. A liquid crystal phase includes a nematic liquid crystal phase, a smectic liquid crystal phase and a cholesteric liquid crystal phase. Of these liquid crystal phases, a nematic phase is employed most popularly in practical liquid crystal display devices. Display modes for liquid crystal displays include a TN (twisted nematic) mode, an STN (super-twisted nematic) mode, DS (dynamic scattering) mode, a guest-host mode, an ECB (electrically controlled birefringence) mode, etc.
Many liquid crystal compounds are known and include those which are still being studied at present. However, no liquid crystal substance capable of being singly filled in a liquid crystal cell to be used by itself is known presently. This is for the following reasons. It is desirable that liquid crystal substances for display devices have a broadest possible mesomorphic temperature range including room temperature at which display devices are used most frequently. It is also desirable that they must be sufficiently stable against the environmental influences relating to them and must have satisfactory physical properties required for display materials. However, no single liquid crystal substance satisfying these conditions by itself is known.
At present, therefore, liquid crystal compositions comprising plural liquid crystal substances, and optionally, along with non-liquid-crystalline substances are prepared and put to practical use as display materials.
It is necessary that such liquid crystal compositions are chemically stable against water, light, heat, air, etc. that are Generally present in the surroundings in which they are used and also against electric and magnetic fields. Further, the liquid crystal compounds to be mixed into the compositions are required to be individually, mutually and chemically stable in the surroundings in which they are used.
Liquid crystal compositions must have suitable optical anisotropy and dielectric anisotropy, and also a suitable viscosity and a suitable mesomorphic temperature range, in accordance with the display mode and the devices in which they are used.
The most popular display modes which are generally employed at present, include a TN (twisted nematic) mode having a twist angle of 90 degrees and an STN (super-twisted nematic) mode having a twist angle being between 180 degrees and 270 degrees, in which modes the optical activity and the birefringent property of the liquid crystals are utilized, respectively. Regarding recent liquid crystal display devices for these systems the matters of the greatest importance are the following:
(1) To realize low driving voltage and low power consumption; PA1 (2) To minimize the temperature-dependence of the electrooptical characteristics of liquid crystal materials; and PA1 (3) To realize high-speed response, etc. PA1 (1) A liquid crystal composition comprising a first component consisting of at least one compound selected from compounds of the above-mentioned general formulae (Ia) and (Ib), a second component consisting of at least one compound selected from compounds of the above-mentioned general formulae (IIa) and (IIb), a third component consisting of at least one compound selected from compounds of the above-mentioned general formulae (IIIa) to (IIIh), (IVa) to (IVo) and (Va) to (Vg), and a fourth component consisting of at least one compound selected from compounds of the above-mentioned formulae (VIa) to (VIh) and (VIIa). PA1 (2) A liquid crystal composition according to the above-mentioned item (1), in which the mixing proportions of the first, second, third and fourth components are 3 to 50%, 3 to 35%, 10 to 60% and 10 to 60%, respectively, all by weight relative to the total weight of these four components. PA1 (3) A liquid crystal composition according to the above-mentioned item (1), in which the mixing proportions of the first, second, third and fourth components are 5 to 30%, 5 to 25%, 15 to 40% and 20 to 50%, respectively, all by weight relative to the total weight of these four components. PA1 (4) a liquid crystal display device containing the liquid crystal composition according to any one of the above-mentioned items (1) to (3).
In order to be satisfactory in these matters, liquid crystal materials to be used in liquid crystal display devices are required to have satisfactory characteristics with respect to the threshold voltage, the optical anisotropy, the viscosity, etc. The lowering of the threshold voltage makes it possible to realize low driving voltage and low power consumption, while the reduction in the temperature-dependence of the threshold voltage makes it possible to solve the problem of the insufficient contrast in the practical temperature range at which the display device is used and also to eliminate display failure such as cross-talk.
In display devices, it is necessary to define the product (.DELTA.n.times.d) of the optical anisotropy (.DELTA.n) of the liquid crystal material to be filled in a cell and the thickness (d .mu.m) of the cell at a pre-determined value, depending on the display mode to be applied to these devices, etc. In this case, where a liquid crystal material having a large .DELTA.n is used, the value of d may be small.
The response time (.tau.) in a display device is proportional to the viscosity (.eta.) of the liquid crystal material to be placed in the device and is proportional to the square of the thickness (d) of the cell containing the material. Therefore, if the values of d and .eta. are small, .tau. is noticeably reduced, thereby realizing rapid response. For these reasons, it is much desired that liquid crystal compositions are obtained that have a reduced threshold voltage with reduced temperature-dependence and having suitably elevated optical anisotropy and also a low viscosity.